I. We have studied possible immune and nonimmune host defense against unrelated human mucosal parasitic protozoa. A. Giardia lamblia does not activate the alternative complement pathway. With antibody (from goat or rabbit), however, the parasites are killed by complement via the classical pathway. Without complement, such antiserum inhibits parasite attachment. B. Trichomonas vaginalis activates the alternative complement pathway and is killed by the resultant membrane attack complex. C. Natural resistance factors: G. lamblia is killed by lactoferrin, an iron-binding protein present in human intestinal fluid and polymorphonuclear cells. Lactoferrin binding to trophozoites was shown by indirect immunofluorescence. Killing activity was thermolabile and the parasites were protected by serum. II. Drug Studies: A. Growth of G. lamblia was inhibited by DFMO, an irreversible inhibitor of polyamine biosynthesis. Thus, polyamine biosynthesis appears to be required for growth and may be a chemotherapeutic target. B. Certain quassinoid protein synthesis inhibitors are potent amebacides.